Amine curable epoxy resin formulations are widely used as coatings, adhesives, sealants, and matrices for fiber-reinforced composites. For many applications, a fast rate of cure is desirable.
Many additives have been tested as cure accelerators for epoxy/amine mixtures. Several references teach that additives with phenolic hydroxyl groups are effective with epoxy resins derived from epihalohydrins and active hydrogen compounds, such as bisphenol A epoxy resins. For example, Shechter et al in Industrial and Engineering Chemistry, Volume 48, No. 1, pages 94 to 97, 1956, disclosed that phenol was more effective than aliphatic alcohols in accelerating the reaction of phenyl glycidyl ether with diethylamine. Bowen et al in the American Chemical Society Advances in Chemistry Series, Volume 92, pages 48 to 59, 1970, disclosed that a variety of hydroxyl containing compounds decreased the gel time of a bisphenol A epoxy/triethylenetetramine mixture. Bowen et al disclosed that 4,4'-dihydroxydiphenyl sulfone, glycerin, phenol, tetrabromobisphenol A, and hisphenol A accelerated the cure with a similar degree of effectiveness.
Epoxy compositions containing resorcinol are described in the prior art. For example, Gough et al (in the Journal of Oil and Color Chemists Association, volume 43, pages 409 to 418, 1961), Nagy (in Adhesives Age, pages 20 to 27, April, 1967), and Pattensky (in the American Chemical Society Advances in Chemistry Series, Volume 92, pages 29 to 47, 1970) disclosed that resorcinol and many other phenolic compounds accelerate the cure of glycidyl epoxy/amine mixtures. Markovitz in "Chemical Properties of Crosslinked Polymers," American Chemical Society Symposium 1976, S. S. Labana, Ed., pages 49 to 58 described curable compositions containing cycloaliphatic epoxides, resorcinol and metal salts as coaccelerators. No reference was found to cycloaliphatic epoxide/aromatic amine mixtures containing resorcinol as an accelerator.
In many epoxy/amine formulations, cycloaliphatic epoxides are used as the epoxy component since they impart improved mechanical and thermal properties to the cured compositions. For example, unreinforced castings of bis(2,3-epoxycyclopentyl) ether cured with m-phenylenediamine have tensile strengths and tensile moduli which are among the highest of any thermosetting material. Similarly, as described by McLean et. al. in Report No. 14450 of the National Research Council of Canada, November, 1974, high mechanical properties can be achieved in unreinforced castings made by curing 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexane carboxylate with methylene dianiline. However, resin systems containing bis(2,3-epoxycyclopentyl) ether or 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexane carboxylate cure more slowly with aromatic amines than similar compositions containing hisphenol A epoxy resins. This characteristic limits their utility in composite fabrication processes such as filament winding and reaction injection molding. Thus there is a need for cure accelerators for cycloaliphatic epoxide/amine resin systems. Moreover, in commercial practice it is desirable that the mixture of the accelerator and epoxy resin have good storage stability in the absence of the amine hardener. This characteristic facilitates handling in a production environment.
It has now been found that a select group of phenolic compounds are highly effective cure accelerators for cycloaliphatic/aromatic amine resin systems. Under a fixed cure schedule, the accelerated compositions afford improved properties compared to compositions which do not contain the accelerator, such as higher mechanical properties and/or increased heat deflection temperatures in unreinforced castings.
Further, a method for accelerating the cure of cycloaliphatic epoxide/aromatic amine mixtures at low temperatures has been found which comprises adding a solid solution of a high melting accelerator in a low melting solid cycloaliphatic epoxy resin.